This invention relates to rotating shaft systems which may be subject to periodic excessive vibration, and more particularly to means for controlling excessive vibration and preventing damage therefrom.
A requirement for the proper design of rotating shaft systems is to place the primary bending modes of the shaft system outside the normal operating speed range of the shaft. A common method of doing this is to install bearings at specific locations along the shaft length, thus adding stiffening at those points and thereby raising the critical speed. The critical speed is defined as the rotational speed of a shaft at which an imbalance force coincides with the fundamental or some higher mode of the natural frequency of transverse vibration of the shaft and its attached masses.
The expedient of installing bearings of a type suitable for continuous service, with their accompanying lubricating feed, at a position where bending vibration might intermittently occur raises both the weight and cost of the equipment, and increses the problem of alignment. For this reason, if the bending mode is only such as arises from the normal imbalance of a well-designed shaft system, if it occurs at a speed which must be passed through but is below the speed range of normal operation, and if the system is such that the critical speed is passed through quickly in running up to normal operating speed and equally rapidly while running down, it may be ignored and a bearing at that point omitted. However, this practice is suitable only in situations where all operating conditions are precisely known, where it is possible to pass through the critical speed with sufficient rapidity, and where no unexpected event may occur.
One type of such an unexpected event is the sudden occurrence of a large imbalance in the shaft system, such as when a turbine rotor loses a blade. The spring rates of journal bearings increase significantly with bearing load and a rotating system will have different resonant response when it operates with large imbalances. The normal bearings of the system would be unable to control the shaft vibration, with the likelihood of severe damage to the system.